linux-kernel - Re: [PATCH 0/3] readfile(2): a new syscall to make open/read/close faster

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Date:   Mon, 6 Jul 2020 18:25:41 +0100
From:   Dave Martin <Dave.Martin@....com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     viro@...iv.linux.org.uk, mtk.manpages@...il.com, shuah@...nel.org,
        linux-api@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-man@...r.kernel.org,
        linux-kselftest@...r.kernel.org
Subject: Re: [PATCH 0/3] readfile(2): a new syscall to make open/read/close
 faster

On Sat, Jul 04, 2020 at 04:02:46PM +0200, Greg Kroah-Hartman wrote:
> Here is a tiny new syscall, readfile, that makes it simpler to read
> small/medium sized files all in one shot, no need to do open/read/close.
> This is especially helpful for tools that poke around in procfs or
> sysfs, making a little bit of a less system load than before, especially
> as syscall overheads go up over time due to various CPU bugs being
> addressed.
> 
> There are 4 patches in this series, the first 3 are against the kernel
> tree, adding the syscall logic, wiring up the syscall, and adding some
> tests for it.
> 
> The last patch is agains the man-pages project, adding a tiny man page
> to try to describe the new syscall.

General question, using this series as an illustration only:

At the risk of starting a flamewar, why is this needed?  Is there a
realistic usecase that would get significant benefit from this?

A lot of syscalls seem to get added that combine or refactor the
functionality of existing syscalls without justifying why this is
needed (or even wise).  This case feels like a solution, not a
primitive, so I wonder if the long-term ABI fragmentation is worth the
benefit.

I ask because I'd like to get an idea of the policy on what is and is
not considered a frivolous ABI extension.

(I'm sure a usecase must be in mind, but it isn't mentioned here.
Certainly the time it takes top to dump the contents of /proc leaves
something to be desired.)

Cheers
---Dave